Generally, polymers polymerized by using a single site catalyst show narrow molecular weight distribution and uniform distribution of comonomers, with higher copolymerization activity than that of Ziegler-Natta catalyst. However, due to their narrow molecular weight distribution, the processing would require large energy consumption and be difficult to be performed by using existing equipments, but increasing the processing cost. When the techniques for polymerizing olefin by the use of single site catalyst are analyzed from the viewpoint of conventional commercialized processes, they can be directly applied (in case of a high-pressure solution process) if the solubility of the single site catalyst in solvent is sufficiently high, and significant issues would be stability of catalyst at relatively high polymerization temperature, and removal of catalytic activity during the work-up process after the reactor, as well as separation of impurities and reaction inhibitors during the course of isolating, purifying and recovering the solvent.
In order to ensure processibility as well as improved physical properties of ethylene copolymer which is polymerized by using single site transition metal catalyst, it is advantageous for the copolymer to have broader molecular weight distribution, or molecular weight distribution showing two or more peaks in the molecular weight distribution curve.
In order to manufacture such ethylene copolymer with improved processibility and physical properties, U.S. Pat. No. 4,935,474 discloses a process wherein two or more metallocene catalysts having different reaction rates are used in one reactor. According to this process, however, it is difficult to prepare ethylene copolymer with various density distributions, though polymers having relatively broad molecular weight distribution or bimodal molecular weight distribution could be prepared.
U.S. Pat. No. 3,592,880, EP 057420 and 237294, GB Patent 2020672, or the like disclose slurry-slurry multi-stage polymerization processes; GB Patent 1505017, EP 040992, U.S. Pat. No. 4,420,592, or the like gas-gas multi-stage polymerization processes; and GB Patent 1532231, U.S. Pat. Nos. 4,368,291, 4,309,521, 4,368,304 or the like slurry-gas multi-stage processes. Though WO 9212182 states that two or more stage are possible for gas process in a slurry-gas process, only bimodal molecular weight distribution via a two-stage process is shown, due to catalyst characteristics and accordingly introduction of hydrogen. Examples of the patent suggest production of ethylene copolymer with restricted density of at least 0.930 g/cm3, so that the process implies limitation to produce ethylene copolymer resin of various use, such as film with high impact strength.
WO 1994/17112 proposes a process for preparing ethylene copolymer with broad molecular weight distribution by using metallocene and Ziegler-Natta catalyst in solution polymerization, but this process provides only bimodal molecular weight distribution with limitation in improvement of physical properties of the polymer through the modified process.
U.S. Pat. No. 6,277,931 also discloses a process for polymerizing ethylene having bimodal molecular weight distribution by using heterogeneous catalyst (metallocene and Ziegler-Natta) in a solution polymerization process. However, when heterogeneous catalyst is used in a system, interference between the heterogeneous catalysts or with cocatalyst may occur, so that the reaction would be hardly controlled. The cocatalyst for Ziegler-Natta catalyst may act as catalyst poison or reaction inhibitor against single site catalyst.
WO 2006/048257 proposed a process for ethylene copolymer with broad molecular weight distribution and trimodal molecular weight distribution via three reactors. The process is designed as a slurry-gas process wherein high density polyethylene with high molecular weight is partially synthesized in a prepolymer reactor prior to the slurry process, and then slurry and gas phase process are carried out to provide ethylene copolymer having trimodal and broad molecular weight distribution. However, the high molecular weight portion with high density may result in deleterious effect on impact strength of film from the aspect of overall resin.
U.S. Pat. No. 6,372,864 proposed a process for preparing ethylene copolymer having satisfactory physical properties and processibility by using single site catalyst containing phosphinimine ligand(s) in two stirred tank reactors. According to the process, however, a large amount of comonomer should be employed in the process to give low density, due to the catalyst property, and thus comonomer would remain in the final polymer product to cause problems in odor and hygiene.
U.S. Pat. No. 6,995,216 suggests a process for preparing ethylene copolymer having broad molecular weight distribution by using single site catalyst containing crosslinked indenoindolyl ligand(s) in a multi-stage or multiple reactor(s). But the process does not consider complete mixing of the reactants through the multi-stage, so that the polymer synthesized in each stage may have disadvantages due to insufficient mixing.